The present invention is directed to a method of making a more economical and distortion-free silicon nitride product suitable for use as one or more cutting tools by hot pressing with significantly reduced need for subsequent shaping.
Hot pressing of ceramic starting materials has been known for some time (see Refractory-Materials, Alper, Vol. 5, III, "High Temperature Oxides", 1970, pp. 184-189, for an explanation of the typical hot pressing process and equipment). The hot pressing sequence usually involves placing a loose particulate powder mixture or semidense pressed block of the powder mixture into a pressing assembly and heating the assembly while applying pressure to the mass sufficient to densify and fuse the particles to a desired degree. Typically, the pressing assembly is a cylindrical die closed by end plungers or pistons, one or both of such end plungers or pistons being forceably moved by platens of a press to apply pressure to the mass within the assembly. The cylinder and end plungers are close fitting and are typically constructed of graphite. A refractory insulation shell is wrapped about the cylindrical die assembly and heat is applied thereto by induction coils or by resistance heating. For making a silicon nitride comprising ceramic, hot pressing is typically carried out in the temperature range of 1500.degree.-1850.degree. C., the pressures employed usually are in the range of 2000-7000 psi, and the time period usually comprises 5-180 minutes. The resulting density for silicon nitride so hot pressed is usually in the range of 3.15-3.40 gm/cm.sup.3.
The conventional hot pressing technique has not achieved a desirable level of productivity and economy. At best, such art has attempted to simultaneously hot press a plurality of aligned loose powder volumes, each volume separated from the other by a fully dense, thick and rigid spacer effective to transmit pressure forces like a wall of the pressure assembly. Each such spacer limited the capacity of the assembly to increase productivity and in many cases occupied more space than the bodies to be pressed (see U.S. Pat. Nos. 3,535,132; 3,632,708; and 3,264,388).
In the interest of economy and productivity, if the hot pressing sequence were to be employed for the simultaneous pressing of a plurality of stacked, cold compacted, flat ceramic plates (particularly large diameter plates on the order of a 6 inch diameter or greater), several problems would be encountered. First, a temperature gradient is created across the lateral width of the plates which results in a corresponding viscosity gradient. Such viscosity gradient causes the pressing assembly to apply a nonuniform pressure distribution across and through the mass of the plates. Secondly, there exists a drag force (a friction force between the pressing assembly walls and the plate sides) which also contributes to a nonuniform pressure distribution across the lateral width of the plates. These two factors together cause material transport under the hot pressing conditions which in turn results in "dishing" or severe distortion of the flat plates in their fully densified condition. Lacking dimensional accuracy such hot pressed bodies would require expensive reconditioning to redefine for use.
In only one instance has the prior art attempted to simultaneously hot press a plurality of silicon nitride components. In British Pat. No. 1,405,171, a number of cold compacted preforms are placed in a single layer within a pressing assembly, each preform having a thickness generally equal to its width. Each preform is separated from all others by a release agent. No greater than two layers are used. The problems overcome by this invention would not be experienced in the application of this British patent. Side wall drag would be insufficient to promote distortion since there is little difference in movement between layers; the preforms do not contact the die wall and the thickness to width ratio is only 1:1. Material transport cannot take place as a result of pressure and thermal gradients because there is little relative movement between layers, little or no side will drag, and the thickness to width ratio is only 1:1. The disclosure thus fails to appreciate the need for a unique stacking sequence that would eliminate dishing in the hot pressing of multiples of billets having a thickness to width ratio of 1:3-1:40.